1. Field
The present invention relates generally to seismic imaging and more particularly to velocity model correction.
2. Background
Seismic surveying is used to characterize subsurface formations and in particular for locating and characterizing potential hydrocarbon reservoirs. One or more seismic sources at the surface generate seismic signals that propagate through the subsurface, reflect from subsurface features, and are collected by sensors. Raw data is generally in the form of travel times and amplitudes, which must be processed in order to obtain information about the structure of the subsurface.
Typically, processing includes inversion of the collected time information to produce a velocity model of the subsurface structure. Because there are usually multiple velocity solutions that satisfactorily explain any given set of time data, it is not always known whether the velocity models accurately depict the subsurface structure. In this regard, even the most geophysically advanced techniques for inverting seismic data may result in velocity models that, when used as the basis of subsurface imaging techniques, produce geologically implausible seismic images. For example, a seismic image might show crossing beds, which cannot represent the real geological structure. Also, there may be reflection events that are believed to have simple structure but are not simple in a seismic image. This often occurs for reflection events in fault shadows and for basement reflection events where they are beneath a complex overburden.
Geophysical inversion techniques that rely on flattening image gathers often cannot correct the image. In noisy conditions and complex geology, it can be hard to see events on the prestack gathers, and, even if the events can be seen, it is sometimes hard to modify the velocity model so that they become flat. Moreover, even if velocity model changes lead to flat gathers, the image can still turn out to be implausible because a range of velocity models often will flatten the gathers but produce significantly different structure in the image.